1. Technical Field
This invention relates in general to no-ID data storage disk drives, and in particular to a system and method for storing and identifying cylinder addresses in no-ID disk drives in such a way that reduces track seek time.
2. Description of the Background Art
In a magnetic storage system such as a disk drive, digital information is magnetically stored upon a surface of a magnetic medium in a set of concentric circular patterns called "tracks". The digital information is represented by selectively polarizing the surface of the disk. When this information is read back from the disk, the magnetic polarization of the medium is sensed and converted to an electrical output signal. The read and write operations are performed by read/write electronics in conjunction with a read/write head which "flies" over the surface of the rotating disk and provides an output signal.
Typically, storage disks of a disk drive are stacked in a "disk stack" (also known as "disk pack") which are mounted for rotation together on a single spindle. Each side of each disk in the disk pack has a surface which is usually used to store information.
Each surface of a disk in the disk pack is usually exposed to at least one head responsible for reading and writing information on that particular surface. Typically, all the magnetic heads which are mounted on actuator arms move in tandem radially over the surfaces of the disk so that they are all at the same disk radius at the same time.
In order to accurately move a magnetic head to a desired track and position the head over that track a servo system is utilized. The servo system performs two distinct functions known as a "seek" ("access") function and a "track following" function. During the "seek" operation the servo system moves a read/write head to a selected track from a previous track or from a park position. When the head reaches the desired track, the servo system begins a "track following" operation in which it accurately positions the head over the centerline of the selected track and maintains the head in that position as successive portions of the track pass by the head. It is important to notice that during a seek operation the actuator arm where the head is mounted must be moved as fast as possible so as to minimize the time required for that operation. Since the seek time is one of the most important factors used in measuring the overall performance of a disk drive, it is very important to minimize the time it takes for performing that operation as much as possible.
In order to read and write data from the correct location in the disk pack, the data regions in the disk pack are identified by a cylinder address, head address and sector address (CHS). A "cylinder" identifies a set of specific tracks on the disk surfaces in the disk pack which lie at equal radii and are, in general, simultaneously accessible by the collection of heads. The head address identifies which head can read the data and therefore identifies the disk that the data is recorded on. Each track within a cylinder is further divided into "segments" where each segment comprises one or more "sectors" for storing data.
Many modern disk drives also use a concept known as zone bit recording (ZBR), as taught by Hetzler in U.S. Pat. No. 5,210,660, in which the disk surface is divided into radial zones and the data is recorded at a different data rate in each zone. The addition of zones requires expansion of the cylinder, head, sector (CHS) identification scheme to a zone, cylinder, head, sector (ZCHS) identification scheme.
Some disk files have servo information only on a dedicated surface on one disk in a disk stack. However, many modern disk drives use a servo architecture known as "sectored servo", as taught by Hetzler, U.S. Pat. No. 5,210,660 where servo information is interspersed with the data stored on each disk surface. The servo sectors in sectored servo architecture (also referred to as "sector servo" architecture) contain positioning data on each track to help the magnetic head stay on that track. This latter approach is preferred because it can be implemented at low cost without extra components beyond those required for storing data. Furthermore, it provides the servo information at the data surface being accessed, thereby eliminating all thermal sources of track misregistration (TMR). The use of either sectored servo or dedicated servo surface architectures and the implementation of either of the two are well known to those skilled in the requisite art.
Fixed block architecture (FBA) is a common configuration used to format both dedicated servo disk files and sector servo disk files. In a FBA formatted disk file, each disk track is divided into a number of segments and each segment is divided into one or more sectors for storing servo information, identification information (ID), and data.
A typical segment 9 of a track on a FBA formatted disk utilizing sectored servo architecture is illustrated in FIG. 1. The segment 9 comprises sequentially a servo sector 10, identification (ID) region 11 and a data sector 12. Servo sector 10 further comprises information such as write-read and speed field 15, address mark (AM) field 16 and position error signal (PES) field 17. ID region 11, which is written onto the disk during the format operation, contains specific information concerning succeeding data sector 12 which can be used during normal operation, either writing or reading, to identify the succeeding data sector 12. ID region 11 typically comprises a read/write and speed field 18, VCO sync field 19, encoder/decoder flush field 20, sync byte 21, and ID and CRC field 22. Data sector 12 typically comprises fields 23-26 which correspond to ID fields 18-21, and data and ECC field 27.
Recently, a new method and system has been developed to increase the capacity of disk drives known as the no-ID format and the disk drive systems utilizing no-ID format are commonly referred to as no-ID disk drive systems. This format has been taught by Hetzler in a co-pending U.S. patent application Ser. No. 07/727,680 (SA9-91-022), filed Jul. 10, 1991, and assigned to the assignee of the present invention.
Briefly stated, in no-ID disk drives implementing a sectored servo architecture, a "full track number identifier" in the position field in the servo region of each sector is used in combination with a defect map to uniquely identify the requested data sectors and thereby completely eliminate the use of ID regions.
In doing so, bad sectors are mapped out of the disk file by means of a defect map. At disk format time, each sector is written to and read from to determine whether it is usable or defective. Clusters of defective sectors are marked bad by recording in the look-up table the sector location identifier of the first bad sector in the cluster and the quantity of consecutive bad sectors in the cluster. During the read/write operations, the disk file performs logical block address (LBA) to physical block address (PBA) conversion (logical to physical sector conversion) by searching the look up table for an entry having a value less than or equal to the requested logical sector location identifier. If none is found, the physical block address is equal to the logical block address. If an entry is found, the corresponding offset representing the quantity of consecutive bad sectors is extracted from the defect map and added to the logical block address of the requested sector to produce the physical block address for that sector. Then the physical block address is converted into cylinder, head and sector (CHS) value or zone, cylinder, head, and sector value corresponding to the logical block address of the requested sector.
Since the ID region is no longer available in no-ID disk drives, cylinder address which is usually a long pattern, has to be stored in some other location on the disk and acquired by a different means. The only possible location for storing cylinder address is in the servo sector.
However, there is a serious disadvantage associated with storing a cylinder address in a servo sector. Information in a servo sector is generally written at a relatively low frequency to ensure, among other things, track to track accurate servo alignment. Writing long cylinder address in a servo sector means that (1) the maximum servo seek velocity is substantially reduced and (2) the disk is no longer being utilized efficiently. The former problem is due to the fact that the maximum velocity of the servo seek is directly proportional to the length of the cylinder address stored in each servo sector on each track. The longer the cylinder address, the longer the track seek time. This is due to the fact that the track seek velocity must be slow enough to ensure that, as the head passes over a given track, the complete cylinder address written in at least one sector of that track is completely read before the head is moved over to another track. Otherwise, only part of the cylinder address can be read by the head which leads to serious servo address errors.
Since average seek time is one of the main factors used in comparing similar disk drives with each other, substantial efforts are being made to find ways to reduce average seek time. Therefore, allowing longer seek time due to long cylinder address would be contradictory to that goal. As a result, there is a great need for an invention that can substantially eliminate overall performance degradation associated with storing cylinder address in the servo sector in no-ID disk drives while improving servo track seek time and disk space utilization.